The present invention relates to a method for recovering hydrogen chloride from chlorine based waste, and more specifically, it relates to a process for recovering high quality hydrogen chloride from an acid gas generated by the combustion of chlorine based solid waste such as polyvinyl chloride, polyvinylidene chloride and chloroprene rubber and chlorine based waste containing organic chlorine compounds such as a chlorine based organic waste liquor to be discharged from plants for producing a vinyl chloride monomer, dichlorobutene, chlorobenzene and the like.
Organic chlorine compounds have such a number of excellent properties that they are stable with reduced deterioration and further, easily available and economical and accordingly, polyvinyl chloride representing them is being used in a large quantity all over the world. However, its used material is simultaneously generated in a large quantity and further, chlorine based organic waste liquors are generated at the time of its production. On that account, from the standpoint of preservation of environment and effective utilization of resources, the technology of treatment of these types of waste is demanded from all angles and a number of proposals have been made.
For example, Japanese Patent Publication (Kokai) No. Hei 8-267049/1996 discloses a method for subjecting waste containing chlorine based polymeric compounds to heat treatment, allowing the generated hydrogen chloride gas to be absorbed in water, neutralizing the resulting hydrogen chloride aqueous solution with an alkali agent such as sodium hydroxide and calcium hydroxide and evaporating water to obtain a chloride. This technique is comparatively simple and can render hydrogen chloride harmless. However, it uses a large amount of a useful alkali agent in neutralizing hydrogen chloride and consumes much energy in evaporating water and thus, is not economical and, in addition, the recovered product is a chloride whose value added has not been high.
Japanese Patent Publication (Kokai) No. Hei 11-286687/1999 discloses a method for obtaining carbides and a hydrogen chloride gas by thermal decomposition of chlorine-containing plastics. It is shown that the carbon component in the waste is formed into carbides which can be used as the fuel of cement and the like and the chlorine component is formed into hydrogen chloride which can be used as the feedstock in the oxychlorination reaction of ethylene with hydrochloric acid and thus, this method is reasonable. However, the dechlorination treatment at the time of thermal decomposition is not complete and as the result, part of chlorine remains in the carbides and there is a fear of material corrosion and product contamination by the chlorine to be generated in use and its migration into a discharged gas and further, floating foreign substances derived from the carbides, carbon monoxide, a carbon dioxide gas, a sulfurous gas, and sometimes, foreign gaseous components such as a chlorine gas are present in the recovered hydrogen chloride and accordingly, these substances have had to be purified and removed in some cases.
Japanese Patent Publication (Kokai) No. Hei 11-158319/1999 discloses a method of heating waste plastics containing polyvinyl chloride and plasticizers to obtain an organic solid substance and a hydrogen chloride gas and, in the purification of the resulting hydrogen chloride gas, cooling the resulting hydrogen chloride gas to separate the plasticizers, allowing the cooled gas to be absorbed in water to form hydrochloric acid and then, allowing organic components in the hydrochloric acid adsorbed on active carbon to remove them. According to this method the quality of hydrochloric acid can considerably be improved to broaden its use. However, a large amount of active carbon is necessary and its regeneration is difficult and thus, this method is not economical. In addition, the adsorption capacity of active carbon of inorganic substances and hydro-philic low molecular weight organic substances are small and thus, the quality of the obtained hydrochloric acid could not be said sufficient. Further, the organic solids are expected to be utilized as fuels but they contain a chlorine component which sometimes has to be purified and eliminated in use.
Japanese Patent Publication (Kokai) No. Hei 11-80746/1999 discloses a method for recovering high purity hydrochloric acid which comprises thermally decomposing waste plastics containing polyvinyl chloride to obtain a solid or liquid fuel and a hydrogen chloride gas, allowing the resulting hydrogen chloride gas to be absorbed in water to obtain hydrochloric gas, adding hydrogen peroxide to this hydrochloric acid to decompose and remove organic substances. However, the oxidative decomposition of organic substances by hydrogen peroxide does not have much effect on high molecular weight organic substances, and the action of removing inorganic substances is not much expected. Further, in addition to the cost of hydrogen peroxide, generation of a chlorine gas by hydrogen peroxide is anticipated and thus, the quality of hydrochloric acid could not be said sufficient.
The target component to be dealt with by the combustion of chlorine based waste is a chlorine gas. Its concentration varies depending on the combustion conditions, and the chlorine gas to be generated relates to corrosion of the equipment and as the countermeasure, Japanese Patent Publication (Kokai) No. Hei 6-180104/1994 discloses a method for preventing the corrosion of equipment by injecting a sulfurous acid gas into a combustion gas of a chlorine-containing fuel. This method is effective for anticorrosion of equipment but unreacted sulfurous acid gas and the sulfuric acid mist formed by reaction accompany a hydrogen chloride gas and are absorbed in water to come to sulfuric acid and sulfurous acid to bring about reduction in the quality of hydrochloric acid. Further, the absorption efficiency of unreacted sulfurous acid gas into water is low and thus, the sulfurous acid gas migrates from absorption equipment into a discharged gas and as the result, there is a fear of causing air pollution. Further, this Patent Publication does not show the purification of the hydrogen chloride gas and hydrochloric acid.